Response Of Jiaojiang Estuary To Coastal Reclamation

Sea enclosing and coastal reclamation is an important way to accommodate the increasing need of space for urban development in coastal areas, and it has brought considerable social and economic benefits. Reclamation projects will change the morphology as well as the flow and sediment transport of the estuaries and coasts. As a result, a series of challenges for ecological environment protection, port and waterway dredging, flood control, etc. have been arisen. The negative effects on sustainable development of estuaries and coasts caused by large-scale reclamation of medium and low tidal flat are particularly significant. The adjustment of estuary after reclamation is a very complicated process. The research on the complex response of estuary to reclamation can provide a theoretical basis for the scientific planning and construction of reclamation projects.The basic parameters of nonuniform sediment transport in the Jiaojiang estuary are obtained at first. And on this basis, a depth-averaged 2D mathematical model for tidal current and nonuniform sediment transport in estuaries is developed. Based on the fractal theory, the hydraulic geometry for tidal estuaries is proposed. Through the analysis of the field data, theoretical research and numerical simulation, the response of flow and sediment transport as well as the sea-bed evolution in the Jiaojiang estuary to coastal reclamation are studied in this paper.To reflect the unsteady properties of sediment transport in estuaries, a time-dependent formula with a logarithmic form for nonuniform sediment transport capacity in the Jiaojiang estuary is proposed. The formula has the unified form for fractional and total transport capacity and the self-adaptability to uniform sediment while the mean settling velocity for non-uniform sediment is proved to be equal to the geometric average of settling velocities for sediment fraction. The theoretic formula of saturation recovery coefficient for nonuniform sediment is accomplished by elementary function, and it makes the calculation more convenient. The formula for settling velocity used in this paper can adapt to various passing-flow states. The influence of sediment concentration on settling velocity is considered in the form of kinetic viscosity, and this makes the formula can apply to calculate the fall velocity of particles in any sediment concentration. The flocculating settling is taken into account for the fine sediment in estuaries. Based on the incipient probability of nonuniform sediment, the formulas of critical shear stress for erosion and deposition are obtained. The nonuniform characteristics of sediment and the viscous force of fine particles are taken into account in the critical erosion shear stress.The critical deposition shear stress reflects the increase of the settling probability caused by the molecular attraction quantitatively, and also considers the nonuniform characteristics of particles. The coefficient of erosion rate determined by scouring test of drill core samples takes the influence of seabed consolidation state into account.A depth-averaged two-dimensional (2D) mathematical model for tidal flow and non-uniform sediment transport in estuaries is established based on finite element method (FEM) in this paper, and two alternative sediment transport modules are developed using the aforementioned theoretical results of nonuniform sediment. Module 1 is based on the time-dependent formula of sediment transport capacity and the theoretical formula of saturation recovery coefficient. The nonuniform grain composition and the unsteady transport of estuarine sediment are taken into account. Module 2 is based on the formulas of critical shear stress for erosion and deposition of non-uniform sediment. The influence of viscous force to erosion and flocculating settling of fine nonuniform particles are considered. The model is tested by experimental data and validated by the measured data from Jiaojiang estuary, and the results turned out that the model has a good adaptability to the simulation of tidal flow and nonuniform sediment transport.The hydraulic geometry and tidal volume in tidal estuaries display a power functional relation, and it has a fractal characteristics. Based on the fractal theory and the principle of dimensional homogeneity, taking the relative flood-tide (ebb-tide) volume as the observation scale to describe relative cross-section area, and the relative unite flood-tide (ebb-tide) volume to describe the relative water depth, the hydraulic geometry for tidal estuaries are proposed. The formulas of relative water depth and relative unite flood-tide (ebb-tide) volume is applicable to open seas where there is no concept of cross-section. The formulas of hydraulic geometry for the Jiaojiang estuary and Oujiang estuary are established. The results indicate that the formulas have good applicability in tidal estuaries. The fractal dimension of hydraulic geometry reflects the stability of estuary and the relative effects of hydrodynamic condition during flood-tide and ebb-tide. The value of fractal dimensions are more close to 1 when the estuary is relatively stable or the hydrodynamic force is stronger.The tidal current and sediment transport of the Jiaojiang estuary in different reclamation stage are simulated by the present mathematical model for tidal flow and non-uniform sediment transport. Results show that the flow velocity increases significantly in new extend part of estuary, while the flow velocity decreases in estuarine channel and part of the Taizhou Bay. The area of maximum velocity goes to seaward with the extension of the river mouth. The areas of tidal flat and Taizhou bay are reduced drastically due to the large-scale reclamation, and this lead to the decrease of both sediment supply and hydrodynamics in the Taizhou Bay and Jiaojiang River. As a result, the transport capacity of sediment is deduced, and the suspended sediment concentration (SSC) decreases. In new extend part of estuary, the SSC increases. The turbidity maximum zone of the Jiaojiang estuary is seaward as the river mouth is extended to the sea due to reclamation.In order to investigate the response of seabed erosion and deposition to cluster reclamation in Jiaojiang estuary, a series of numerical simulations are carried out. According to the results, erosion occurs in most part of estuarine channel while a small amount of deposition occurs in mouth-bar region in the situation of present coastline. When the planned reclamation is imposed, a serious deposition will occur in estuarine channel and mouth-bar region while the new extend part of estuary caused by reclamation will be eroded. Compared with the combined effect of each single reclamation on erosion and deposition, the effect of the whole cluster reclamation is larger, that is to say, there is a nonlinear effect. With the increase in the area of reclamation, the hydrodynamics of estuarine channel and mouth-bar region weakens and therefore sediment transport capacity decreases, then the thickness of deposition increases in these areas. Special attention should be paid to the effect of cluster reclamation on seabed evolution.